Recent Changes in Hammerhead

Taxonomy

Glenn A. Fines wrote: "I just have a quick question
for someone who may have more literary resources available than myself. I am
working on a presentation on Hammerhead sharks. All of the literature I have
(1993 and earlier) classify this group as Family Sphyrnidae with two genera Sphyrna
and Eusphyra. However, it was mentioned to me that this may have been
changed recently and the family Sphyrnidae was reverted to a single genus Sphyrna
with a subgenus Eusphyra within the family Carcharhinidae. If anyone has
an up to date classification, or knows where I can find one, of the hammerheads
I would greatly appreciate it."

A lot has happened in hammerhead systematics and taxonomy in recent years. As
yet, the nomenclatural dust has not settled, but I will try to bring you (and
anyone else who's interested) up to speed as best I can.

The eight extant species of hammerhead shark (Sphyrna couardi was
synonymised under S. lewini by McEachran and Serret in 1986) are among
the most easily recognised of sharks. To a greater or lesser extent, all
hammerheads are characterized by unique lateral expansions of the orbital and
nasal regions of the chondrocranium and associated tissues, forming a
distinctive double-bladed 'cephalofoil'.

Other than this unusual headgear, the hammerheads are very similar in overall
form to the whaler sharks of the family Carcharhinidae (as established by Jordan
& Evermann in 1896). Most modern workers (such as Bigelow & Schroeder
1948, Gilbert 1967, Compagno 1973, 1979, 1988, Bass, D'Aubray, & Kistnassamy
1975, Nelson 1976, 1984, Cadenat & Blanche 1981, among others) therefore
regarded the hammerheads as a distinct family, the Sphyrnidae sensu Gill
1872, closely allied with — and almost certainly derived from — the Carcharhinidae. Compagno (1979, 1988) noted that the hammerheads are
morphologically most similar to the carcharhinid Scoliodon laticaudus
(the spadenose shark) and hypothesised that the hammerhead sharks may share a
relatively recent common ancestor with that genus. The earliest hammerheads in
the fossil record of which I am aware are undescribed teeth in the collection of
David Ward from middle Eocene deposits of Tunisia — but the Scoliodon/Loxodon/Rhizoprionodon
to Sphyrna transition is extremely difficult to determine precisely, as
the teeth of these genera are very similar (D. Ward, pers comm.).

It had been assumed (Compagno 1979, 1988) that the hammerheads represent a
monophyletic group (including all the descendants of a common ancestor) and that
the cephalofoil evolved gradually, starting from a non-hammerheaded
carcharhinoid ancestor — most probably similar to Scoliodon in overall
form. According to this common-sense scenario, the basic 'hammer' design evolved
only once in sharks, the modestly-cephalofoiled bonnethead shark (Spyrna
tiburo) was the earliest of the hammerheads to diverge and the
boomerang-nosed winghead shark (Eusphyra blochii — I'll get into the
matter of genera and subgenera momentarily) the most recent.

Sketches from my laboratory notebook, showing development of the hammer in
an embryo versus a subadult Scalloped Hammerhead (Sphyrna lewini)

Based on my own
dissections of the heads of embryo, juvenile, and adult hammerheads of several
species, I seemed to see evidence that the hammer did indeed evolve gradually
from smaller to larger. The sequence of hammer evolution — from none to modest
to extreme — seemed to make perfect sense and to be fully supported by
morphological and developmental evidence. But it now seems that our collective
common sense was simply wrong. Using mtDNA sequences, Martin (1993) provided
powerful evidence that the hammerheads are indeed a monophyletic group but that
the sequence of divergence was precisely opposite from that which had long been
assumed: the 'extremely' hammer-headed winghead shark was the earliest of the
group to diverge and the comparatively ridge-headed bonnethead the most recent.
Further, Martin's analysis suggested that the relative width of the cephalofoil
in various species of hammerheads (expressed as a percentage of body length)
does not show a clear trend in the in-between species of hammerheads. This
pattern suggests that the hammerhead cephalofoil appeared rather suddenly, fully
realized, and was only later modified in various ways in response to differing
selective pressures.

In a popular article originally published in Sea Frontiers
(it has
appeared in several forms since then), I tried to answer the question of why
hammerhead sharks have such bizarre heads — that is, what 'function' the hammer
might serve these sharks. As I noted, there are basically two schools of thought
on the matter: 1) hydrodynamic benefit versus 2) sensory enhancement. Evidence
for the former school includes: hammerheads are among the most negatively
buoyant of sharks (Baldridge 1970, 1972); a relatively constant ratio of
anterior planing surfaces (there seems to be an inverse relationship between the
size of the hammer and the pectoral fins, resulting in a fairly constant total
surface area of these planing surfaces), the wing-like cross-section of the
hammer (obvious on inspection), possibly acting as a canard; and the apparent
autonomic control hammerheads seem to have over the trailing edge of the
cephalofoil (only recently published, see Nakaya 1995), noting that other sharks
often seem to avoid hammerheads in competitive contexts, possibly because of
their enhanced maneuverability. Evidence for the latter school includes: early
arrival at olfactory attractants (possibly due to wide spacing of the nares,
allowing hammerheads to sample a wider portion of the water column) and
observations by myself and others of hammerheads using their hammer in
mine-sweeper fashion, apparently to locate prey buried in the sediment using the
electrosensitive organs (ampullae of Lorenzini) distributed over much of the
undersurface of the head.

As a specialist in the behavioral ecology of elasmobranchs, I was (and still
am) somewhat biased in favor of the second school of thought — that the
hammerhead 'hammer' was originally more a matter of sensory enhancement than
hydrodynamic benefit. Using the example of the 'extremely' hammer-headed winghead shark, I inferred that the 'cephalofoil' in this species seemed
downright unwieldy, apparently developed to a point beyond any conceivable
hydrodynamic benefit, yet the nares (which, unlike other species of hammerhead,
are closer to the midline of the hammer than the tips) and ampullae (though not
as widely spread over the undersurface of the hammer as in other species) are
more widely separated than in most non-hammerheaded sharks. I further noted that
the hammerhead cephalofoil is a complex structure that probably serves more than
one function. In conclusion, I therefore ranked the functions of the
hammerhead cephalofoil (arranged from most to least significant) as: 1)
increasing electrosensitive acuity, 2) increasing scent-tracking efficiency, 3)
increasing maneuverability, and 4) increasing lift. Although I had gotten the
sequence of hammerhead divergence bass-ackward, Martin's (1993) findings do seem
to support my bias that the hammer originally evolved as a sensory structure,
only later taking advantage of hydrodynamic advantages accruing from lateral
expansion of the head. The way I now interpret the sequence of hammerhead
divergence is as follows: The hammer appeared relatively suddenly (possibly the
result of relatively minor changes in epistatic genes or perhaps — but less
likely — transporons changing the rate or pattern of developmental events during
epigenesis), resulting in a hammer similar to that we see in the modern winghead
shark (which can be up to 44% of the standard length, with the nares located
closer to the midline of the hammer than the tips and the ampullae of Lorenzini
clustered mostly near the center of the hammer); in later 'models', the hammer was
decreased in size, the nares migrated to the tips of the hammer and the pattern
of ampullae become more complex and widespread over the undersurface of the
hammer (see Gilbert 1967, p 70, for an illustration of this). I therefore submit
that the hammerhead 'hammer', once it appeared, became fixed under conditions
of enhanced prey-locating ability (which would be expected to result in enhanced
reproductive fitness) and was later modified to take advantage of hydrodynamic
benefits as a secondary consideration. Therefore, in the development of the
hammerhead cephalofoil, sensory enhancement was initially more important than
hydrodynamic benefit.

Gilbert (1967) placed all hammerheads in the genus Sphyrna and recognized
three subgenera within the genus, 1) Eusphyra for S.
blochii, 2) Platysqualus for S. corona, S. media, S.
tiburo, and S. tudes, and 3) Sphyrna for S. couardi
(which is now a junior synonym of S. lewini), S. lewini, S.
mokarran, and S. zygaena. Compagno (1988) erected Mesozygaena
as a new subgenus for S. tudes, S. corona, and S. media,
and retained S. tiburo in the subgenus Platysqualus and S.
lewini, S. mokarran, and S. zygaena in the subgenus Sphyrna.
Comapgno (1979, 1988) ranked Eusphyra Gill 1862 as a genus, based on the
following characters: lateral blades of cephalofoil very narrow and wing-like;
nostrils tremendously enlarged, width 0.75 to 0.88 of internarial width and
nearly twice mouth width; prenarial nodules present. Dingerkus (1986) objected
to the elevation of Eusphyra to generic status, on the grounds that it
implies that Sphyrna is paraphyletic (does not include all the
descendants of a common ancestor — as in the case of 'reptiles', as usually
defined, excludes birds and mammals, yet all three classes share a common
ancestor). This argument is inherently circular (essentially complaining
"Compagno can't split hammerheads into two genera, because then we won't
have one genus of hammerheads"). It is worth noting in passing that the
species rank is the only 'natural' taxon that can be clearly defined among
sexually-reproducing organisms; all other taxa are arbitrary and may be regarded
as most useful when they best serve their twin purposes (a basis for generalization
in comparative studies and an information storage system — see Mayr 1981 for a discussion of this). As I understand the implications of the
cladogram presented in Martin (1993), since the winghead shark does not occur
among the other hammerheads, assigning it to a separate genus (in this case, Eusphyra)
does NOT render the genus Sphyrna paraphyletic. The real question, then,
is: Are the differences noted by Compagno (1979, 1988) sufficiently great to
warrant the winghead shark a genus distinct from other hammerheads? Nelson
(1994) regarded Eusphyra as a subgenus; Shirai (1996) retained it as a
full-fledged genus. As all biological taxa above the species rank have no
'reality' in the natural world, this is largely a subjective decision.

Until recently, the Scalloped
Hammerhead (Sphyrna lewini) was thought to be distinct from the
Whitefin Hammerhead (Sphyrna couardi). Despite its name, the
Whitefin Hammerhead does not have white or white-tipped fins. On the
basis of broadly overlapping morphometric characters, this species was
synonymized under Sphyrna lewini by John McEachran and Bernard
Seret in 1987 (Cybium, 11[1]: 39-46)

I have no claims to being a systematist or taxonomist; like many shark
enthusiasts, I regard these matters occasionally interesting but mostly they are
just plain necessary — important scientific disciplines that enable us to be
certain we are all discussing the same species or group within the same
conceptual (usually evolutionary) framework. I am by inclination a 'lumper',
preferring to simplify matters by grouping things into as few categories as
serves my purposes. While I feel that Compagno sometimes divides groups of
elasmobranchs too finely (in his 1984 FAO catalogue, Sharks of the World,
9 out of 30 [30%] families and 49 out of 98 (50%) genera are monotypic,
represented by only a single species; given the extremely low genetic
variability and tremendous genetic sluggishness of sharks in general, combined
with their rather conservative structure, this — in my admittedly inexpert
opinion — seems somewhat excessive), but Compagno's work is by all accounts very
good and his conclusions have been borne out so often that I am very reluctant
to dismiss his ideas without VERY compelling evidence to the contrary. To my
mind, placing the winghead shark in its own genus, distinct from the other
hammerheads, serves to reflect its unique cephalic features and early
evolutionary divergence from the common ancestor of the group as indicated by
Martin's (1993) mtDNA data. So, pending further evidence to the contrary, I'm
prepared to go along with Compagno on this point for the time being. You may
choose not to; that is your right and privilege as a free-thinking adult.

And finally, the familial status of the hammerhead sharks appears to be no
longer warranted. Naylor (1992) examined 17 presumed loci of structural proteins
in 37 species of carcharhinoid shark. Despite only limited overlap between the
genetic markers used and including many species not examined by Lavery (1992),
both Naylor and Lavery's results fully support Compagno (1979, 1988)'s
contention that the hammerhead sharks form a monophyletic group, closely allied
with the genus Rhizoprionodon (sharpnose sharks). Unfortunately, neither
Naylor nor Lavery tested the spadenose shark — which Compagno (1979, 1988) hypothesized
is the sister taxon to the hammerheads — to determine its phylogenetic position among the other carcharhinoids tested. But Naylor's (1992)
genetic sequence data also revealed something rather surprising: the family
Carcharhinidae (as presently defined and constituted) is not monophyletic UNLESS
the hammerheads are included within that family. To put this more bluntly:
despite its many unique features, the Sphyrnidae is not a valid family.
Surprising as this finding may be, Compagno (1988) explored this very
possibility, proposing an alternate phylogenetic classification to allow for
inclusion of the hammerheads within the family Carcharhinidae. Naylor (1992)
advocated accepting Compagno's alternate hypothesis, pending further data.
Nelson (1994) and Shirai (1996) have adopted this taxonomic arrangement. Martin
(1993) further strengthens the argument in favor of accepting Compagnos (1988)
classificatory scheme; this approach is followed here with the exception of
placing S. media and S. tudes in the same subgenus as S. tiburo,
as suggested by Martin's cladogram.

A Provisional Cladistic Classification of Hammerhead Sharks within
the Family Carcharhinidae
(Modified after Compagno 1988)

Several of Naylor's (1992) analyses suggest a close affinity between the
little-known Loxodon macrorhinus and (of all things) the Reef
Whitetip Shark, Triaenodon obesus, which is attractive because the former appears to
be a rather base-level carcharhinid and a close evolutionary affinity with
Triaenodon may help explain the latter's many intriguing triakid-like
characters. The phylogenetic position of Triaenodon has been the subject of
much recent taxonomic fiddling; Randall, Allen & Steene (1990) and Myers
(1991) placed it in the family Hemigaleidae, which also seems to fit the
genetic data fairly well IF the molecular systematic trees are collapsed via
semi-strict consensus and/or character weighting. Further study of the
matter is indicated. [Up to Loxodon]

**

Based on independent cladistic analyses by Compagno (1988) and Naylor
(1992), the validity of this genus is now rather doubtful, and the genus
Carcharhinus as it now stands will almost certainly require being split into
two or more genera. The cladograms of these workers agree in many respects,
the 'Carcharhinus' sharks falling fairly readily into two broad groups that
correspond reasonably well to Springer's (1950, 1951) scheme of dividing
these sharks into smooth-backed Carcharhinus and ridge-backed Eulamia.
Springer's (1950, 1951) scheme may therefore be a good place to start
untangling these commercially important but tough to tell-apart sharks. [Up
to Carcharhinus]

I hope this helps.

Cheers,

— R. Aidan Martin

Originally posted to SHARK-L Feb 24, 1998.

UPDATE: Since this was originally posted, I have developed a novel
phylogenetic hypothesis that redefines Carcharhinidae in a way that favors
retention of Sphyrnidae as a valid family and seems fully consistent with
all data available to me. This hypothesis is presently being tested as part
of a collaborative morphological/molecular systematic project which I am not
at liberty to discuss at this time.